Variable gain amplifier and a large scale integrated circuit installed thereof applicable to processing signals

ABSTRACT

Variable gain amplifier and a LSI including the variable gain amplifier that expands an output voltage range or control voltage range without increasing power consumption. The variable gain amplifier includes a voltage-current conversion circuit; a variable gain unit; and a voltage output unit. The voltage-current conversion circuit outputs a first positive current and a first negative current in proportion to an input voltage. The variable gain unit inputting the first positive current and the first negative current controlled by a control signal and outputs four output currents including a second positive current, a third positive current, a second negative current and a third negative current. The voltage output unit inputs either the second positive current and the second negative current or the third positive current and the third negative current Iop 3 , and outputs a positive output voltage and a negative output voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from Japanese PatentApplication No. 2003-185350, filed on Jun. 27, 2003, which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a variable gain amplifier and a largescale integrated circuit (LSI) installed thereof applicable toprocessing signals, such as amplifying optical signals received from anoptical disk or amplifying transmitted and/or received signals in amobile communication terminal. In particular, the present inventionrelates to a variable gain amplifier and an LSI that substantiallyexpands an output voltage range (i.e., a dynamic range) and also expandsa gain control voltage range without increasing the power consumption ofan operation circuit of a high frequency waveform.

A variable gain amplifier is widely used to vary its voltage gain basedon a control signal. As an example application of a variable gainamplifier at a high-frequency operation area, a variable gain amplifierhas been used to amplify various signals, such as signals received froman optical disk or intermediate frequency (IF) signals in atransmitting/receiving unit of a mobile communication device.

Since such a high-frequency operation circuit needs to increase anoperational current value in order to perform a quick operation of eachcircuit element, power consumptions of the high-frequency operationalcircuit generally tend to increase for performing a quick operation ofelements, such as transistors and resistors. Consequently, the largerincrease of the power consumption of the circuit becomes a seriousproblem in accordance with a circuit configuration becoming morecomplicated and larger in size.

To reduce the increase of the power consumption, it has been proposed toconstruct the circuit configuration by vertically stacking elements oftransistors and resistors. This vertical stacking can increase signalprocessing functions per a unit current. Accordingly, it becomespossible to increase a signal processing function per unit current. As aresult, it becomes possible reduce the number of current paths thatincrease the power consumption of a variable gain amplifier. Thus, itbecomes possible to reduce currents between a source voltage terminalV_(DD) to a grounding terminal GND.

With reference to FIGS. 14 and 15, the above-mentioned problems of aconventional variable gain amplifier will be explained. FIG. 14 is ablock diagram for illustrating a conventional variable gain amplifier.Thus, a conventional variable gain amplifier is comprised of avoltage-current conversion circuit 1, a variable gain unit 4, a voltageoutput unit 5, and a common-mode feedback unit 6. The voltage-currentconversion circuit 1 provides two differential current outputs Iop1 andIon1 in proportion to a voltage difference of two input voltages Vip andVin. The differential current outputs Iop1 and Ion1 have a respectiverelation represented by numerical formula (1).Iop1=+g _(m)(Vip−Vin)+1_(h)Ion1=−g _(m)(Vip−Vin)+1_(h)  (1)Iop1−Ion1=2g _(m)(Vip−Vin)In formula (1), g_(m) is a proportion constant, and I_(B) is a biascurrent that has a constant value.

The variable gain unit 4 inputs the two differential currents Iop1 andIon1 and outputs two differential input currents Iop2 and Ion2. Thedifferential current outputs Iop1 and Ion1 in the formula (1) arerespectively multiplied by A and (1−A), and are added so that the twodifferential input currents Iop2 and Ion2 satisfy the followingnumerical formula (2). Here, it is supposed that a coefficient value Aof the variable gain unit 4 is variable in accordance with an externalcontrol voltage Vc to the variable gain unit 4, and the coefficientvalue A meets a condition that A∝Vc.Iop2=A*lop1+(1−A)*Ion1Ion2=A*Ion1+(1−A)*Iop1  (2)

At this time, a difference between the differential output currents Iop2and Ion2 of the variable gain unit 4 is represented by the followingnumerical formula (3) by using the formulas (1) and (2).Iop2−Ion1=(2A−1)(Iop1−Ion1)=2g(2A−1)(Vip−Vin)  (3)Thus, the difference between the differential output currents Iop2 andIon2 in the variable gain unit 4 is proportional to a difference(Vip−Vin) of the two input voltages Vip and Vin to the voltageconversion circuit 1.

Further, the differential output currents Iop2 and Ion2 from thevariable gain unit 4 are supplied to the voltage output unit 5.Consequently, from which two output voltages Vop and Von are generateddue to voltage drops of the respective loads Z1 and Z2. It is alwayssupposed that the load Z1 is equal to the load Z2. Thus, Z1=Z2=Z. Therespective output voltages Vop and Von from the voltage output unit 5are represented by the following formula (4).Vop=Z*(Iop2−1c)+vcomVon=Z*(Ion2−1c)+Vcom  (4)Here, two output bias current Ic are provided from the common-modefeedback circuit 6. Accordingly, a difference of the output voltages Vopand Von is represented as follows.Vop−Von=Z*(Iop2−Ion2)=4g _(m) Z(2A−1)(Vip−Vin)  (5)From the above formula (5), it is understood that a difference betweenthe output voltages Vop and Von from the voltage output unit 5 isproportional to a voltage difference between the two input voltages Vipand Vin. Thus, the voltage difference Vi=Vip−Vin. Accordingly, when thecoefficient value A of variable gain unit 4 is varied by the controlsignal Vc to the variable gain unit 4, it becomes possible to realize avariable gain amplifier where its voltage gain is controlled by thecontrol signal Vc.

The common-mode feedback unit 6 compares a mean value Vcom of the twooutput voltages Vop and Von with an external reference voltage Vref. Byamplifying a voltage difference between them and by adding the amplifiedvoltage difference to the respective output bias current Ic, a feed-backloop is constructed through the voltage output unit 5. By doing this,the mean value Vcom of the two output voltages Vop and Von is alwayscontrolled to be equal to the external reference voltage Vref.

FIG. 15 illustrates a detail circuit configuration of the variable gainamplifier illustrated in FIG. 14. When two differential input voltagesVip and Vin are supplied to a voltage-current conversion unit 3 involtage-current conversion circuit 1, two differential output currentsIop1 and Ion1 are outputted from voltage-current conversion circuit 1.Variable gain unit 4, voltage output unit 5 and common-mode feedbackunit 6 are constructed by the same current path between a power sourcevoltage VDD and a grounding potential GND. By vertically stackingelements of transistors and resistor so as to construct a plurality offunction blocks in the same current path between a power source voltageVDD and a grounding potential GND, it becomes possible to reduce acurrent consumption.

Two output currents Iop1 and Ion1 from voltage-current conversion unit 1satisfy the above described formula (1). Thus, they are represented asfollows.Iop1=I _(g) +g _(m1) ViIon1=I _(B) −g _(m1) Vi  (6)In formula (6), the two output currents Iop1 and Ion1 are inputted toeach of transistors M1 to M4 in the variable gain unit 4. Here, eachsize of the respective transistors M1 to M4 is supposed to be equal,i.e., M1=M2=M3=M4=M, and a trans-conductor parameter K₂ is supposed tobe represented by formula (7).

$\begin{matrix}{K_{2} = {\frac{1}{2}\mu\; C_{ox}\frac{W}{L}}} & (7)\end{matrix}$Here, μ is the hall mobility ratio, Cox is a capacity of an unit area ofgate, W is a channel width, and L is a channel length.

When the control voltage Vc is equal to a difference between two controlvoltages Vc1 and Vc2 in variable gain unit 4, i.e., Vc=Vc1−Vc2, eachdrain current of the respective transistors M1 to M4 in variable gainunit 4 are approximately represented by formula (8), respectively.

$\begin{matrix}{{I_{1} = {\frac{Iop1}{2} - {{Vc}\sqrt{2K_{2}{Iop1}}}}}{I_{2} = {\frac{Iop1}{2} + {{Vc}\sqrt{2K_{2}{Iop1}}}}}{I_{3} = {\frac{Iop1}{2} + {{Vc}\sqrt{2K_{2}{Ion1}}}}}{I_{4} = {\frac{Iop1}{2} - {{Vc}\sqrt{2K_{2}{Ion1}}}}}} & (8)\end{matrix}$

Accordingly the two output currents Iop2 and Ion2 of the variable gainunit 4 are represented by formulas (9) and (10), respectively.

$\begin{matrix}{{{Iop2} = {{I_{2} + I_{4}} = {\frac{{Iop1} + {Ion1}}{2} + {{Vc}\sqrt{2K_{2}}\left( {\sqrt{Iop1} - \sqrt{Ion1}} \right)}}}}{{Ion2} = {{I_{2} + I_{4}} = {\frac{{Iop1} + {Ion1}}{2} + {{Vc}\sqrt{2K_{2}}\left( {\sqrt{Ion1} - \sqrt{Iop1}} \right)}}}}} & (9)\end{matrix}$Iop2−Ion2=2Vc√{square root over (2K ₂)}(√{square root over(Iop1)}−√{square root over (Ion1)})  (10)

By assuming that I_(B) is larger than, the difference of the twodifferential output currents Iop1 and Ion1 are calculated and areapproximately represented by formula (11).

$\begin{matrix}{{{Iop2} - {Ion2}} = {2{Vc}\sqrt{\frac{2K_{2}}{I_{B}}}g_{ml}{Vi}}} & (11)\end{matrix}$Since the difference of the two differential output currents Iop1 andIon1 is represented as formula (11), it is understood that the outputcurrents of the variable gain unit 4 vary its current gain with a valueof the control voltage Vc.

When the differential currents Iop2 and Ion2 of formula (11) aresupplied to the voltage output unit 5, two output voltages Vop and Vonare represented by formula (12).

$\begin{matrix}\begin{matrix}{{Vo} = {{{Vop} - {Von}} = {\left( {{Vop} - {Vcom}} \right) - \left( {{Von} - {Vcom}} \right)}}} \\{= {{Z\left( {{Iop2} - {Ic}} \right)} - {Z\left( {{Ion2} - {Ic}} \right)}}} \\{= {Z\left( {{Iop2} - {Ion2}} \right)}} \\{= {2{Zvc}\sqrt{\frac{2K_{2}}{I_{B}}}g_{ml}{Vin}}} \\{= {\beta \cdot {Vc} \cdot {Vin}}} \\\left( {\beta = {{2g_{ml}Z\sqrt{\frac{2K_{2}}{I_{B}}}} = {{const}.}}} \right)\end{matrix} & (12)\end{matrix}$Accordingly, the output voltage (Vo=Vop−Von) is proportional to theinput voltage Vi of the variable gain amplifier, and its voltage gainvaries with the control voltage V_(c). Transistors M5 and M6 in thecommon-mode feedback unit 6 construct a cascade circuit for increasingan output resistance.

The variable gain amplifier shown in FIG. 14 or 15 have problems anddisadvantages as explained below. We should consider the conditions ofhow the variable gain amplifier shown performs normal operations. Atfirst, each of transistors M1 to M4 in the variable gain unit 4 shouldbe operated in their respective saturation areas. If each of transistorsM1 to M4 does not operate in a saturation area, a low resistancecomponent at each of source terminals of the respective transistors M1to M4 is impossible to see from output terminals Vop and Von.Accordingly, the characteristic of the output voltage Vo cannot besatisfied by above-mentioned formula (12).

Assuming that a control voltage Vc1 is constant (Vc1=constant), and thecontrol voltage Vc2 is larger than the control voltage Vc1 (Vc2>Vc1),the following conditions (13) should be satisfied in order that each oftransistors M1 to M4 normally operate in a saturation area.Vop≦Vcl−V _(Tp)Von≦Vcl−V _(Tp)  (13)The above conditions (13) indicate that each upper limit voltage of therespective output voltages Vop and Von from the voltage output unit 5 isrestricted by the value of the control voltage Vcl in the variable gainunit 4.

Each of the lower limit voltages of the output voltages Vop and Von arealso restricted. In order to have a sufficiently high output resistance,each of transistors M5 to M 8 of the common-mode feedback unit 6 shouldbe operated in a saturation area, and the following conditions (14) mustbe satisfied.Vop≧V _(DS5(sat.)) +V _(DS7(sat.)) =const.Von≧V _(DS6(sat.)) +V _(DS8(sat.)) =const.  (14)V_(DS(sat.)) may have a voltage value of approximately from 0.2V to0.4V.

Further, in order to operate transistors M9 and M10 in their respectivesaturation areas, the source voltages Vs1 and Vs2 of transistors M1 toM4 in the variable gain unit 4 must satisfy the following conditions(15).V _(S1) ≦VDD−V _(DS9(sat)) =const.V _(S2) ≦VDD−V _(DS10(sat)) =const.  (15)Suppose that the control voltage Vc2 in the variable gain unit 4 reachesto a high voltage so that the respective transistors M1 and M4 becomecut-off at the maximum gain. The maximum value of the control voltageVc2 max must satisfy formula (16).Vs1=V _(C2 max) +V _(rp) ≦VDD−V _(DS9(sat.))Vs2−V _(C2 max) +V _(Tp) ≦VDD−V _(DS10(sat.))  (16)

By assuming that each V_(DS9(sat.)) of the respective transistors hasthe same voltage, the above-mentioned formulas (13) to (16) aresummarized as shown in formula (17).max{V _(O-P-P)}+max{V _(C) _(—) _(P-P)}=(Vo max−Vo min)+(V _(C2) max−V_(C1))=VDD−3V_(DS(sat.)) =const.  (17)Formula (17) represents that a sum of the maximum amplitude value of theoutput voltage Vo from the variable gain amplifier and a variablevoltage range of the control voltage has a respective constant value.Thus, the maximum amplitude value (18) and the variable voltage range ofthe control voltage (19) are respectively explained as follows.max{V _(O) _(—) _(P-P)}  (18)max{V _(C) _(—) _(P-P)}  (19)Accordingly, it is understood that the conventional circuitconfiguration is extremely restricted by the formula (17). In spite ofthis it is preferable to keep wider ranges for both the output voltageand the control voltage in order to achieve quick operations for a highfrequency circuit. The range of the maximum output voltage and the rangeof the control voltage becomes a trade-off relationship. Accordingly,since it is impossible to increase both the output voltage and thecontrol voltage at the same time, the maximum amplitude of outputvoltage Vo should become a small value when the variable range of thecontrol voltage Vc(=|Vc2−Vc1|) is expanded.

If the maximum amplitude of output voltage Vo becomes a small value, itis a disadvantage for a signal noise ratio (S/N), since an apparentnoise level against a signal level becomes a large value. Further, if itis attempted to keep a variable gain range required by a specificationunder such a condition that the variable range of control voltage Vc issmall, the amplifier circuit operates under an extremely unstablestatus, since a large gain variation occurs due to disturbances, such asa noise.

Generally, it is required for a variable gain amplifier applicable to asignal processing circuit for receiving signals from an optical disk,such as a digital versatile disk (DVD) or a compact disk (CD) andapplicable transmitting and receiving IF signals for a mobilecommunication terminal to keep a wider range of variable gain. If therange of the variable gain becomes small, it is necessary to construct amultiple stages of stacked amplifiers. This is an extreme disadvantagewhen constructing a variable gain amplifier into a compact and smallsize. Further, it generates another problem of high power consumption.If the variable gain range becomes wider, gain sensitivity against thecontrol signal also becomes higher. Normally, the control signal is anexternal input signal to a variable gain amplifier. Accordingly, thecontrol signal is easily affected due to disturbances, such as noise.Thus, even when attempting to keep a wider gain range within a narrowercontrol voltage gain, the conventional variable gain amplifier becomesan unstable circuit with a large variation of gain due to noises.

Since transistors comprising a circuit need to keep a certaindrain-source voltage for a necessary operation, a stacking configurationof transistor elements restricts the circuit operations within a limitedrange of a power voltage. Even when it keeps operation, an operationalvoltage range of an internal circuit with including amplitude of maximumoutput voltage, i.e., a dynamic range, becomes small. When the signalvoltage amplitude becomes small, a circuit becomes very succeptible todisturbances, such as noise. This is also a serious problem. Inparticular, since a required variable gain range for a variable gainamplifier is generally wide, gain sensitivity will become high when acontrol voltage range becomes small. Consequently, output voltageamplitudes of the variable gain amplifier will become unstable againstdisturbances that enter into a control terminal.

SUMMARY OF THE INVENTION

A variable gain amplifier according to an embodiment of the presentinvention, comprising: a voltage-current conversion circuit configuredto output a first positive current and a first negative current inproportion to an input voltage, respectively; a variable gain unitconfigured to supply four output currents that are obtained byprocessing the input first positive and negative currents under acontrol of a gain factor A (0<A<1) controlled by a control signal,wherein the four output currents include a second positive currentobtained by multiplying the first positive current by the factor A, athird positive current obtained by multiplying the first positivecurrent by the factor (1−A), a second negative current obtained bymultiplying the first negative current by the factor A and a thirdnegative current obtained by multiplying the first negative current bythe factor (1−A); and a voltage output unit configured to output apositive output voltage and a negative output voltage by inputtingeither one groups of the second positive current and the second negativecurrent or the third positive current or the third negative current.

A light receiving signal processing LSI according to an embodiment ofthe present invention, comprising: an analog signal processing unitinstalled with the variable gain amplifier, such as defined in claim 1,in which the input voltage may include at least one optical signalvoltage generated from an optical receiver; a digital signal processingunit configured to process digital signals converted from analog signalssupplied from the analog signal processing unit; a decoder configured todecode the converted digital signals; and a processing unit configuredto perform signal processing calculations in the analog signalprocessing unit, the digital signal processing unit, and the decoder,respectively.

A transmitting/receiving signals processing LSI applicable to a mobilecommunication terminal, comprising: an analog signal processing unitinstalled with the variable gain amplifier, such as defined in claim 1,in which the input voltage may include at least one signal received at amobile communication terminal as the input voltage; a digital signalprocessing unit configured to process digital signals converted fromanalog signals supplied from the analog signal processing unit; adecoder configured to decode the converted digital signals; and aprocessing unit configured to perform signal processing calculations inthe analog signal processing unit, the digital signal processing unit,and the decoder, respectively.

A mobile communication terminal according to an embodiment of thepresent invention comprising: a radio controller configured to convertand amplify received radio frequency digital signals into intermediatefrequencies, and to convert and amplify transmitting signals into highfrequency signals; a control unit configured to control operations ofthe mobile communication terminal; an audio control unit configured toperform a bandwidth extension process and a bandwidth compressionprocess in accordance with a data rate received from the control unit;and an audio input/output unit configured to amplify output analogsignals and input analog signals supplied to a speaker from a microphonein the audio input/output unit; wherein the radio controller includes avariable gain amplifier, comprising: a voltage-current conversioncircuit configured to output a first positive current and a firstnegative current in proportion to a positive and a negative inputvoltage, respectively; a variable gain unit configured to supply fouroutput currents obtained by processing first positive current and firstnegative current controlled by a gain factor A (0<A<1) controlled by acontrol signal, wherein the four output currents include a secondpositive current obtained by multiplying the first positive current bythe factor A, a third positive current obtained by multiplying the firstpositive current by the factor (1−A), a second negative current obtainedby multiplying the first negative current by the factor A, and a thirdnegative current obtained by multiplying the first negative current bythe factor (1−A); and a voltage output unit configured to output apositive output voltage and a negative output voltage by inputtingeither the second positive current and the second negative current orthe third positive current and the third negative current.

An optical disk apparatus, comprising: an optical disk drive sectionincluding an optical receiver configured to project a laser beam on anoptical disk and convert the laser beam reflected from the optical diskinto an electric signal, and an optical head amplifier configured toamplify reproduced data through the optical receiver; a system processorsection configured to control operations of the optical disk apparatus;a data RAM section configured to store the reproduced data, a videodecoder section configured to decode the video data stored in the dataRAM section; an audio decoder section configured to decode the audiodata stored in the data RAM section; a D/A and data reproducing sectionconfigured to convert the decoded video and audio data into an analogvideo and an audio signal, respectively; and a monitor and a speakerconfigured to display the reproduced video data and to reproduce thereproduced audio data, respectively; wherein the optical head amplifierincludes a variable gain amplifier, comprising: a voltage-currentconversion circuit configured to output a first positive current and afirst negative current in proportion to a positive input voltage and anegative input voltage, respectively; a variable gain unit configured tosupply four output currents obtained by processing the first positivecurrent and first negative current controlled by a gain factor A (0<A<1)controlled by a control signal, wherein the four output currents includea second positive current obtained by multiplying the first positivecurrent by the factor A, a third positive current obtained bymultiplying the first positive current by the factor (1−A), a secondnegative current obtained by multiplying the first negative current bythe factor A, and a third negative current obtained by multiplying thefirst negative current by the factor (1−A); and a voltage output unitconfigured to output a positive output voltage and a negative outputvoltage by inputting either the second positive current and the secondnegative current or the third positive current and the third negativecurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof this specification, illustrate various embodiments and/or features ofthe invention and together with the description, serve to explain theinvention. Wherever possible, the same reference number will be usedthroughout the drawings for same or like parts.

FIG. 1 is a functional block diagram of an exemplary circuit diagram fora variable gain amplifier in which the methods and apparatus of anembodiment of the present invention may be implemented.

FIG. 2 is an exemplary circuit configuration for variable gain amplifierillustrated in FIG. 1.

FIG. 3 is a functional block diagram of another exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 4 is an exemplary circuit configuration for variable gain amplifierillustrated in FIG. 3.

FIG. 5 is a functional block diagram of a further exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 6 is a functional block diagram of still another exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 7 is a functional block diagram of still further exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 8 is a functional block diagram of another exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 9 is a functional block diagram of another exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIGS. 10A–10D illustrate exemplary circuit configurations of a voltageoutput unit in variable gain amplifier of an embodiment of the presentinvention.

FIG. 11 is a functional block diagram of another exemplary circuitdiagram for a variable gain amplifier in which the methods and apparatusof an embodiment of the present invention may be implemented.

FIG. 12 is a functional block diagram for light received from an analogsignal processing unit for signal processing and mobile communicationtransmitting/receiving signal processing LSI (large scale integratedcircuit) used in the variable gain amplifier illustrated in FIG. 11.

FIG. 13( a) is a circuit diagram of the analog signal processing unit 12illustrated in FIG. 12. FIG. 13( b) and FIG. 13( c) are waveformsthereof.

FIG. 14 is a functional block diagram illustrating a conventionalvariable gain amplifier.

FIG. 15 is a circuit configuration of variable gain amplifierillustrated in FIG. 14.

FIG. 16 is a block diagram illustrating a functional construction of amobile communication terminal to which the variable gain amplifierconsistent with the present invention is applied.

FIG. 17 is a block diagram illustrating details of a mechanism of a diskdrive section of an optical disk apparatus in which the variable gainamplifier consistent with the present invention is applied.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are usedthroughout the drawings to refer to same or like parts. As illustratedin FIG. 1, an exemplary embodiment of a variable gain amplifier 100consistent with the present invention includes a voltage-currentconversion circuit 1, a variable gain unit 4, a voltage output unit 5and a common mode feed back unit 6. The voltage-current conversioncircuit 1 includes a positive-negative voltage generating unit 2 thatgenerates a positive input voltage Vip and a negative input voltage Vinfrom an input voltage and a voltage-current conversion unit 3 thatinputs the positive input voltage Vip and the negative input voltage Vinand outputs a first positive current Iop1 and a first negative currentIon1 in proportion to the potential difference between the positive andnegative input voltages Vip and Vin.

The first positive current Iop1 and the first negative current Ion1 aresupplied to the variable gain unit 4. In the variable gain unit 4, thesupplied currents are controlled by a gain factor A (0<A<1) that iscontrolled by a control signal Vc. Thus, the variable gain unit 4generates a second positive current Iop2 that is obtained by multiplyingA times of the first positive current Iop1, a third positive currentIop3 that is obtained by multiplying (1−A) times of the first positivecurrent Iop1, a second negative current Ion2 that is obtained bymultiplying A times of the first negative current Ion1, and a thirdnegative current Ion3 that is obtained by multiplying (1−A) times of thefirst negative current Ion1. Thus, the variable gain unit 4 generatesfour output currents Iop2, Ion2, Iop3 and Ion3.

Among the four output currents Iop2, Ion2, Iop3 and Ion3, either one ofa pair of the second positive current Iop2 and the second negativecurrent Ion2 or a pair of the third positive current Iop3 and the thirdnegative current Ion3 is supplied to the voltage output unit 5 in orderto output a positive output voltage Vop and a negative output voltageVon from the voltage output unit 5. In FIG. 1, a pair of the secondpositive current Iop2 and the second negative current Ion2, and a pairof the third positive current Iop3 and the third negative current Ion3are connected to a ground terminal.

FIG. 2 illustrates a circuit configuration of the embodiment of thevariable gain amplifier 100 shown in FIG. 1. An output current of thevariable gain unit 4 in FIG. 2 is represented by formulas (20).Iop2=I ₁ Ion2=I ₄ lop3=I ₂ Ion3=I ₃  (20)

Assuming that the value of Vc1 is fixed, and that Vc=Vc1−Vc2, Thefollowing equations are obtained.

$\begin{matrix}{{I_{1} = {\frac{Iop1}{2} - {{Vc}\sqrt{2K_{2}{Iop1}}}}}{I_{2} = {\frac{Iop1}{2} + {{Vc}\sqrt{2K_{2}{Iop1}}}}}{I_{3} = {\frac{Ion1}{2} + {{Vc}\sqrt{2K_{2}{Ion1}}}}}{I_{4} = {\frac{Ion1}{2} - {{Vc}\sqrt{2K_{2}{Ion1}}}}}} & (21)\end{matrix}$

When I1 and I4 are supplied to the voltage output unit 5, the outputvoltage Vo is represented by formula (22).

$\begin{matrix}\begin{matrix}{{Vo} = {{Vop} - {Von}}} \\{= {\left( {{Vop} - {Vcom}} \right) - \left( {{Von} - {Vcom}} \right)}} \\{= {Z\left( {{I1} - {I4}} \right)}} \\{= {Z\left( {\frac{{Iop1} - {Ion1}}{2} - {{VC}\sqrt{2K_{2}}\left( {\sqrt{Iop1} - \sqrt{Ion1}} \right)}} \right)}} \\{= {g_{m}{Z\left( {1 - {{Vc}\sqrt{\frac{2K_{2}}{I_{B}}}{Vi}}} \right)}}}\end{matrix} & (22)\end{matrix}$

Accordingly, a gain of output voltage at the control voltage Vc isvariable.

One of the advantages of the embodiment of the present invention is thata variable range value of control voltage Vc2 can maintain width whencontrol voltage Vc1 remains a constant. Under the circuit configurationillustrated in FIG. 2, drain terminals of the transistors M2 and M3 areconnected to a ground terminal. Accordingly, it never affects an outputresistance of the voltage output unit 5. Consequently, a voltage valueof control voltage Vc2 is not restricted by an operation voltage rangeof voltage output unit 5. Since the control voltage Vc2 can reduce in sofar as that transistors M1 and M4 do not enter a cut-off state, itbecomes possible for a variable voltage range of the control voltage Vc2to be largely separate from the control voltage Vc1 against earthterminal.

Generally, the variable gain amplifier operates at bi-quadrant. Ifcontrol voltage Vc2 is larger than control voltage Vc1 (Vc2>Vc1), thegain of the variable gain amplifier becomes negative. Accordingly, avoltage value of Vc2 is used within a range from the ground terminal tothe control voltage Vc1. However, when the respective size oftransistors M2 and M3 are made smaller than each of transistor size oftransistors M1 and M4, the gain of the variable gain amplifier does notbecome negative, even under the condition Vc2>Vc1. Accordingly, itbecomes possible to further expand a variable voltage range of thecontrol voltage Vc2.

FIG. 3 is a conceptual block diagram illustrating another embodiment ofthe variable gain amplifier consistent with the present invention. Inthe above mentioned embodiment in FIG. 1, currents Iop3 and Ion3 fromthe variable gain unit 4 are connected to a ground terminal. On thecontrary, currents Ion3 and Iop3 in this embodiment illustrated in FIG.3 are respectively added to output currents Ic1 and Ic2 from thecommon-mode feedback unit 6 (Ic1=Ic2=Ic).

FIG. 4 shows a circuit configuration of the variable gain amplifiershown in FIG. 3. As explained above, the currents Ion3 and Iop3 from thevariable gain unit 4 are respectively connected to a drain terminal of atransistor M7 and a drain terminal of transistor M8 in the common-modefeedback unit 6 for addition. At this time, output voltages Vop and Vonfrom the voltage output unit 5 is represented by the following formula(23).Vop=Z(Iop2−(Ic−Ion3))+VcomVon=Z(Ion2−)Ic−Iop33))+Vcom  (23)Accordingly, it is represented as formula (24).

$\begin{matrix}\begin{matrix}{{Vo} = {{{Vop} - {Von}} = {Z\left( {{Iop2} - {Ion2} + {Iion3} - {Iop3}} \right)}}} \\{= {2Z\sqrt{2K_{2}}\left( \sqrt{{IopI} - \sqrt{Ion1}} \right)}} \\{= {2{Zvc}\sqrt{\frac{2{K2}}{I_{B}}}g_{m}{Vin}}}\end{matrix} & (24)\end{matrix}$Formula (24) is similar to formula (12) representing the output voltageof the conventional variable gain amplifier shown in FIG. 15.

In this embodiment illustrated in FIG. 4, currents Iop3 and Ion3including signal elements are used for an addition. Accordingly, itbecomes possible to obtain a wider range of the variable gain comparedto the embodiment illustrated in FIG. 2 in which currents Iop3 and Ion3are wasted by connecting to ground terminal. Further, in thisembodiment, each drain terminal of transistors M2 and M3 in the variablegain unit 4 does not connect to the voltages Vop and Von from thevoltage output unit 5. Accordingly, the voltage Vc2 does not influencethe impedances of the output terminals Vop and Von. Even when controlvoltage Vc2 reaches to the ground potential, the drain terminal voltagesof the transistors M7 and M8 in the common-mode feed back unit 6 neverbecome lower than the voltage Vthp of transistors M2 and M3 in thevariable gain unit 4. Consequently, the control voltage Vc2 can varyfrom the earth potential to the control voltage Vc1. However, even whenthe control voltage Vc2 becomes earth potential, it needs to avoidcutting off of the transistors M2 and M3.

As similar to the above-explained embodiment of the variable gainamplifier consistent with the present invention, if each of thetransistor sizes, i.e., channel width W, of transistors M2 and M3 ismade to be larger than the respective channel width W of transistors M1and M4 in the variable gain unit 4, the gain of the variable gainamplifier does not become negative, even when the condition Vc2>Vc1 ismet. Namely, in order to operate the transistors M9 and M10 in thevoltage current conversion circuit 1 in a saturation area, the upperlimit value of the control voltage Vc2 is represented by formula (25).Vc ₂ ≦VDD−)V _(DS(sat.)) +V _(TP))  (25)

By summarizing the above, the maximum value Vo_(max) and minimum valueVo_(min) of the control voltage Vc2 and the output voltage Vo arerepresented by formulas (26).V _(C2 max) =VDD−(V _(DS(sat.)) +V _(Tp))V_(C2 min)=0Vo _(max) =V _(C1) +V _(Tp)Vo _(min)=2V_(DS(sat.))  (26)

Accordingly, the sum of the maximum amplitude of the output voltage Voand the variable voltage range of the control voltage is obtained byequation (27).max{V _(O) _(—) _(P-P)}+max{V _(C) _(—) _(P-P) }=Vo max−Vo min)+(V_(C2 max) −V _(C2 min))=VDD−3V_(DS(sat.)) +V _(C1)  (27)

By comparing formula (27) to formula (17) for the conventional variablegain amplifier, it is understood that the sum of the maximum amplitudeof the output voltage Vo and the variable voltage range of the controlvoltage in this embodiment consistent with the present invention isincreased by the control voltage Vc1. Namely, the output voltage rangeor the control voltage range is wider by the value of the controlvoltage Vc1 element. Normally, the control voltage Vc1 is higher than atleast VDD/2 in order to keep the output voltage range. Consequently, thecontrol voltage range becomes higher than VDD/2 when compared to theconventional variable gain amplifier. This is a big advantage of theembodiment of the present invention.

By comparing formulas (12) and (24), it is understood that theinput/output characteristics of the variable gain amplifier consistentwith the embodiment of the present invention is the same as one of theconventional variable gain amplifier as illustrated in FIG. 15.Consequently, this means that the variable gain amplifier consistentwith of an embodiment of the present invention expands its maximumoutput amplitude (dynamic range) or its control voltage range withoutchanging the input/output characteristics of the variable gainamplifier. Namely, it becomes possible to realize a variable gainamplifier that is immune to disturbance noises.

FIGS. 5–9 explain modifications of the embodiment of the variable gainamplifier consistent with the present invention. The embodimentillustrated in FIG. 5 is as similar to the embodiment illustrated inFIG. 1 in that both of the third positive current Iop3 and thirdnegative current Ion3 are not supplied the voltage output unit 5 butinstead are supplied to the ground potential side. It is also possibleto connect both of third positive current Iop3 and third negativecurrent Ion3 to a ground potential or a reference potential Vref

The construction of the voltage-current conversion circuit 1 in theembodiment illustrated in FIG. 6 is basically similar to the one in theembodiment depicted in FIG. 3. In the embodiment in FIG. 6, both of thethird positive current Iop3 and the third negative current Ion3 areconnected to the ground potential. A connecting point of resistorelements z comprising the voltage output unit 5 is connected to areference potential Vref1. According to this construction of thevariable gain amplifier, it becomes possible to widely secure thedynamic range or the control voltage range for a variable gain control.

In the embodiment of the variable gain amplifier illustrated in FIG. 7,the third positive current Iop3 and third negative current Ion3 are notconnected to the ground potential but instead to the reference potentialVref as similar to the connecting point of resistor elements of voltageoutput unit 5 in FIG. 6. According to this configuration, it is alsopossible maintain the dynamic range or the control voltage range width.

In the embodiment of the variable gain amplifier illustrated in FIG. 8,the voltage output unit 5 is comprised of two load elements z and acommon-mode feedback unit 6. Each of one end of the two load elements zis connected to the respective output terminal of the two outputcurrents Iop2 and Ion2 among the four output currents Iop2, Iop3, Ion2and Ion3 from the variable gain unit 4 and each of the other end, i.e.,the connecting point, of the two load elements z is connected to anegative input terminal of the second voltage-current converter Gm. Apositive input terminal of the second voltage-current conversion unit isconnected to a reference voltage Vref. The third positive current Iop3and the third negative current Ion3 may be connected to the groundpotential or the reference potential Vref This configuration also canmaintain a wider dynamic range or a wider control voltage range.

The embodiment of variable gain amplifier illustrated in FIG. 9 is alsocharacterized in that the configuration of the voltage output unit 5 issimilar to the embodiment shown in FIG. 8. In FIG. 9, the voltage outputunit 5 is comprised of two resister elements Z1 and Z2, a common-phasefeedback unit 6, a first current adding unit 21 and a second currentadding unit 22. Among the four output currents Ion3, Iop3, Iop2 and Ion2from the variable gain unit 4, two output currents Iop2 and Ion2 aredirectly provided to the respective resister elements Z1 and Z2 in thevoltage output unit 5. A third positive current Iop3 from the variablegain unit 4 is input to the first current adding unit 21 in order tosubtract the output current Ic1 from the common-mode feedback unit 6.The subtracted output current from the first current adding unit 21 isprovided to an output terminal of the negative output voltage Von of thevoltage output unit 5. Similarly, a third negative current Ion3 from thevariable gain unit 4 is input to the second current adding unit 22 inorder to subtract the output current Ic2 from the common-mode feedbackunit 6. The subtracted output current from the second current addingunit 22 is provided to an output terminal of the positive output voltageVop of the voltage output unit 5.

As illustrated in FIGS. 2, 4 and 10A, the load elements in the voltageoutput unit 5 comprising the embodiment of the variable gain amplifierconsistent with the invention are constructed by fixed resistor elementsR1 and R2. FIGS. 10B–10D illustrate another configuration of the loadelements in the voltage output unit 5. It is possible to change theresistor elements in the voltage output unit 5 in FIG. 10A totransistors. Thus, FIG. 10B shows an example of the load elements thatare constructed by N channel transistors. FIG. 10C shows an example ofthe load elements that are constructed by P channel transistors. FIG.10D shows an example of the load elements that are constructed byjunction type transistors of N channel transistor and P channeltransistor.

When the load elements are constructed by the fixed resister elements,it is advantageous for stabilizing operations with a small area size ofthe elements. When the load elements are constructed by transistors, thesetting area size becomes larger than the area for the fixed resistorelements. However, it is advantageous for achieving precise control ofthe gain by controlling a gate voltage. If the technical field applyingthe variable gain amplifier consistent with the invention needs aprecise control, transistors are advantageous for constructing the loadelements.

In FIGS. 2 and 4, the variable gain amplifier is constructed so that asource voltage V_(DD) is used as an operational reference voltage mainlyfor N channel MOS transistors. As illustrated in FIG. 11, it may alsopossible to use a circuit configuration in which the polarity of therespective PMOS transistor and NMOS transistor is converted. Such thatthe variable gain amplifier as shown in FIG. 11 may operate using theground voltage GND as an operational reference. Thus, it achieves thesame advantages as explained in the embodiments illustrated in FIGS. 1and 3.

The present invention is not limited to the above-explained variablegain amplifier. The present invention is also applicable to alarge-scale integrated circuit (LSI) in which an analog signalprocessing unit installs the variable gain amplifier. For example, as anapplication of the variable gain amplifier at a high-frequency, such anLSI is used for an amplifying process for receiving signals from anoptical disk or for a transmitting/receiving unit of intermediatefrequency (IF) in a mobile communication.

As illustrated in FIG. 12, a light receiving signal processing LSI 10 isused for processing light signals reproduced from a disk media 7, suchas a digital versatile disk (DVD) or a compact disk (CD). The lightreceiving signal processing LSI 10 is comprised of an analog signalprocessing unit 11 which installs a variable gain amplifier as explainedin FIGS. 1, 3, 5–9, and 11, a digital signal processing (DSP) unit 12for processing digital signals that are converted from analog signals inthe analog signal processing unit 11, a decoder (DEC) 13 for decodingthe coded signals in the DSP 12, a micro-computer 14 for performingcalculations of the signal processing in the DEC 13 and a static randomaccess memory (SRAM) 15 for storing data. The signal processing LSI 10is constructed by mounting these elements on a circuit substrate.

The signal processing LSI 10 in FIG. 12 may also be used for processingimage signals from a mobile communication terminal. In this case, thesignal processing LSI 10 is used for processing transmitting/receivingsignals. Thus the analog signal processing unit 11 installing thevariable gain amplifier consistent with the present invention receivesan input voltage of the receiving signals from the mobile communicationterminal 8. The DSP 12 processes digital signals that are converted fromthe analog signals by the analog signal processing unit 11. Thus, theimage signals involved in the mobile communication terminal 8 areprocessed as digital signals in the signal processing LSI 10.

FIG. 13( a) explains the construction of the analog signal processingunit 11 illustrated in FIG. 12. Thus, the analog signal processing unit11 is comprised of a variable gain amplifier (VGA) 17 of an embodimentexplained in FIGS. 1, 3, 5–9, and 11, and a waveform equalizationcircuit 18. The VGA 17 receives feedback data from the DSP 12. Thefeedback data relates to signal status. It is desirable that a waveformof the input voltage to the VGA 17 may be a sine wave as shown in FIG.13( b). However, when received signals are obtained from a preciserecording area such as an optical disk or when mobile communicationsignals are transmitted or received in an environment where radioquality is not secured, the waveform is disturbed as shown in FIG. 13(c). In such a case, it needs to perform a waveform shaping to such adisturbed signal voltage to obtain an appropriate sine wave as shown inFIG. 13( b). By providing the appropriate sine wave to the DSP 12, it ispossible to obtain desired reproduced signals from optical receivedsignals or a desired transmitting/receiving signals, in particular,desired receiving signals in a mobile communication terminal.

FIG. 16 is a block diagram illustrating a functional construction of amobile communication terminal 200 to which the variable gain amplifierconsistent with the present invention is applied. Radio frequencydigital signals transmitted from a base station are received by themobile communication terminal 200 through an antenna 201. The receiveddigital signals are converted into intermediate frequencies in a radiocontroller 11. The converted signals are converted using a bandwidthextension process in an audio control unit 17 in accordance with areceived data rate notified from a controller 14. Then, the processedsignals are decoded under a PCM decoding process into analog receivingsignals. The decoded analog signals are amplified in a receivingamplifier in an audio output unit 18and output as an audio through anspeaker in the audio out unit 18. On the contrary, input audio into anaudio input unit 19 through a microphone in the audio input unit 19 isamplified at an appropriate level by a transmitting amplifier in theaudio input unit 19. The amplified input signals are converted to formatsignals by performing a bandwidth compression in accordance with a datarate based on an energy amount of the input audio. The format signalsmodulate carrier signals by using the transmitting data and a spectraldiffusion process is performed by using a diffusion signal allotted tothe respective transmitting channels. The diffused coded transmittingsignals are converted into radio frequency signals by combining thesignals with a local transmitting signal. Effective components of thefrequency converted signals are amplified into high frequency signalsand transmitted to the base station through the antenna of the mobilecommunication terminal. In the mobile communication terminal, thevariable gain amplifier consistent with the present invention is appliedto the amplifier in the radio control 11.

FIG. 17 is a block diagram illustrating details of a mechanism of a diskdrive section of an optical disk apparatus in which the variable gainamplifier consistent with the present invention is applied. Generally,an optical disk apparatus includes a disk drive section, a systemprocessor section, a data RAM section, a video decoder section, an audiodecoder section, and a D/A and data reproducing section. As shown inFIG. 17, the disk drive section contains a motor driving circuit 11, aspindle motor 12, an optical receiver i.e., an optical head 32, a feedmotor 33, a focus circuit 36, a feed motor driving circuit 37, atracking circuit 38, an optical head amplifier 40 and a servo processingcircuit 44. To reproduce the data from the optical disk 10, the opticalhead 32 projects a laser beam on the optical disk 10. An objective lens34 is moved across the radius of the optical disk 10 in accordance withthe driving signal supplied from the tracking circuit 38. The laser beamis reflected from the recording layer on the optical disk 10 andreturned to the optical head 32. The optical head 32 converts the beamreflected from the optical disk into an electric signal. The reproduceddata is transferred and stored in the data RAM section by the systemprocessor section. The stored reproduced data is processed in the systemprocessor section and supplied to the video decoder section and theaudio decoder section, respectively, and are decoded at the respectivedecoders. The D/A and data reproducing section converts the decodedvideo data and the audio data into an analog video signal and an analogaudio signal, and supplies to the video signal and the analog audiosignal to a monitor and a speaker, respectively in order to display thereproduced image and to reproduce the sound, respectively. In theoptical disk apparatus, the variable gain amplifier consistent with thepresent invention is applied to the optical head amplifier 40.

As explained above, the variable gain amplifier consistent with thepresent invention expands the output voltage range, i.e., dynamic range,and the gain control voltage range without increasing the powerconsumption. Thus, it may easily provide a variable gain amplifierhaving a superior gain control characteristics by installing an analogsignal processing unit mounted with the variable gain amplifier into anLSI.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope of andspirit of the invention being indicated by the following claims.

1. A variable gain amplifier, comprising: a voltage-current conversioncircuit configured to output a first positive current and a firstnegative current in proportion to a positive input voltage and anegative input voltage, respectively; a variable gain unit configured tosupply four output currents obtained by processing the first positivecurrent and first negative current controlled by a gain factor A (0<A<1)controlled by a control signal, wherein the four output currents includea second positive current obtained by multiplying the first positivecurrent by a factor A, a third positive current obtained by multiplyingthe first positive current by a factor (1-A), a second negative currentobtained by multiplying the first negative current by the factor A, anda third negative current obtained by multiplying the first negativecurrent by the factor (1-A); a voltage output unit configured to outputa positive output voltage and a negative output voltage by selectivelyinputting either the second positive current and the second negativecurrent or the third positive current and the third negative current;and a common-mode feedback unit configured to provide a feed-back loopthrough the voltage output unit using a voltage difference between thepositive and negative output voltages and an external reference voltage.2. The variable gain amplifier according to claim 1, wherein thevoltage-current conversion circuit includes: a positive-negative inputvoltage generating unit configured to generate a positive input voltageand a negative input voltage by inputting an input voltage and avoltage-current conversion unit configured to output a first positivecurrent; and a first negative current that are respectively inproportion to potential differences between the positive and negativeinput voltages supplied from the positive-negative input voltagegenerating unit.
 3. The variable gain amplifier according to claim 1,wherein two output current terminals among the four output currents arecoupled to a ground potential and not supplied to the voltage outputunit.
 4. The variable gain amplifier according to claim 1, wherein twooutput current terminals among the four output currents are couple to adifferent reference voltage source and not supplied to the voltageoutput unit.
 5. The variable gain amplifier according to claim 1,wherein the voltage output unit comprises two load elements, oneterminal of the respective load elements is connected to the respectiveoutput terminals of the two output currents among the four outputcurrents supplied from the variable gain unit, and the other terminal ofthe respective load elements is connected to each other and the samereference potential as the other two output currents among the fouroutput current supplied from the variable gain unit in order to outputthe positive output voltage and the negative output voltage fromrespective terminals of the voltage output unit.
 6. The variable gainamplifier according to claim 1, wherein the voltage output unit includesload elements comprised of transistors.
 7. The variable gain amplifieraccording to claim 1, wherein the voltage output unit comprises two loadelements, one terminal of the respective two load elements is connectedto the respective output terminals of the two output currents among thefour output currents supplied from the variable gain unit and the otherterminal of the respective two load elements is connected to thenegative input terminal of a second voltage-current converter, and thecommon-mode feedback unit is configured to supply an output current fromthe second voltage-current conversion unit to a positive output terminaland a negative output terminal, wherein a positive input terminal of thesecond voltage-current conversion unit is connected to the referencevoltage.
 8. The variable gain amplifier according to claim 7, whereinthe load elements of the voltage output unit are constructed bytransistors.
 9. The variable gain amplifier according to claim 7,wherein the voltage output unit is configured so one of the second orthe third positive currents among the four output currents from thevariable gain unit, which is not inputted to the voltage output unit, issupplied to a first current adding unit for performing a subtractionfrom an output current of the common-mode feedback unit in order tosupply a first subtracted value to the negative output voltage terminalof the voltage output unit, and one of the second or the third negativecurrents, which is not inputted to the voltage output unit, is suppliedto the second current adding unit for performing a subtraction from anoutput current of the common-mode feedback unit in order to supply asecond subtracted value to a positive output voltage terminal of thevoltage output unit.
 10. A light receiving signal processing LSI,comprising: an analog signal processing unit installed with the variablegain amplifier in which the input voltage may include at least oneoptical signal voltage generated from an optical receiver; a digitalsignal processing unit configured to process digital signals convertedfrom analog signals supplied from the analog signal processing unit; adecoder configured to decode the converted digital signals; and aprocessing unit configured to perform signal processing calculations inthe analog signal processing unit, the digital signal processing unit,and the decoder, respectively, wherein, the variable gain amplifier,comprises: a voltage-current conversion circuit configured to output afirst positive current and a first negative current in proportion to apositive input voltage and a negative input voltage, respectively, avariable gain unit configured to supply four output currents obtained byprocessing the first positive current and first negative currentcontrolled by a gain factor A (0<A<1) controlled by a control signal,wherein the four output currents include a second positive currentobtained by multiplying the first positive current by a factor A, athird positive current obtained by multiplying the first positivecurrent by a factor (1-A), a second negative current obtained bymultiplying the first negative current by the factor A, and a thirdnegative current obtained by multiplying the first negative current bythe factor (1-A), a voltage output unit configured to output a positiveoutput voltage and a negative output voltage by selectively inputtingeither the second positive current and the second negative current orthe third positive current and the third negative current, and acommon-mode feedback unit configured to provide a feed-back loop throughthe voltage output unit using a voltage difference between the positiveand negative output voltages and an external reference voltage.
 11. Atransmitting/receiving signals processing LSI applicable to a mobilecommunication terminal, comprising: an analog signal processing unitinstalled with the variable gain amplifier in which the input voltagemay include at least one signal received at a mobile communicationterminal as the input voltage; a digital signal processing unitconfigured to process digital signals converted from analog signalssupplied from the analog signal processing unit; a decoder configured todecode the converted digital signals; and a processing unit configuredto perform signal processing calculations in the analog signalprocessing unit, the digital signal processing unit, and the decoder,respectively, wherein, the variable gain amplifier, comprises: avoltage-current conversion circuit configured to output a first positivecurrent and a first negative current in proportion to a positive inputvoltage and a negative input voltage, respectively, a variable gain unitconfigured to supply four output currents obtained by processing thefirst positive current and first negative current controlled by a gainfactor A (0<A<1) controlled by a control signal, wherein the four outputcurrents include a second positive current obtained by multiplying thefirst positive current by a factor A, a third positive current obtainedby multiplying the first positive current by a factor (1-A), a secondnegative current obtained by multiplying the first negative current bythe factor A, and a third negative current obtained by multiplying thefirst negative current by the factor (1-A), a voltage output unitconfigured to output a positive output voltage and a negative outputvoltage by selectively inputting either the second positive current andthe second negative current or the third positive current and the thirdnegative current, and a common-mode feedback unit configured to providea feed-back loop through the voltage output unit using a voltagedifference between the positive and negative output voltages and anexternal reference voltage.
 12. A mobile communication terminalcomprising: a radio controller configured to convert and amplifyreceived radio frequency digital signals into intermediate frequencies,and to convert and amplify transmitting signals into high frequencysignals; a control unit configured to control operations of the mobilecommunication terminal; an audio control unit configured to perform abandwidth extension process and a bandwidth compression process inaccordance with a data rate received from the control unit; and an audioinput/output unit configured to amplify output analog signals and inputanalog signals supplied to a speaker from a microphone in the audioinput/output unit; wherein the radio controller includes a variable gainamplifier, comprising: a voltage-current conversion circuit configuredto output a first positive current and a first negative current inproportion to a positive and a negative input voltage, respectively; avariable gain unit configured to supply four output currents obtained byprocessing first positive current and first negative current controlledby a gain factor A (0<A<1) controlled by a control signal, wherein thefour output currents include a second positive current obtained bymultiplying the first positive current by the factor A, a third positivecurrent obtained by multiplying the first positive current by the factor(1-A), a second negative current obtained by multiplying the firstnegative current by the factor A, and a third negative current obtainedby multiplying the first negative current by the factor (1-A); and avoltage output unit configured to output a positive output voltage and anegative output voltage by inputting either the second positive currentand the second negative current or the third positive current and thethird negative current.
 13. An optical disk apparatus, comprising: anoptical disk drive section including an optical receiver configured toproject a laser beam on an optical disk and convert the laser beamreflected from the optical disk into an electric signal, and an opticalhead amplifier configured to amplify reproduced data through the opticalreceiver; a system processor section configured to control operations ofthe optical disk apparatus; a data RAM section configured to store thereproduced data, a video decoder section configured to decode the videodata stored in the data RAM section; an audio decoder section configuredto decode the audio data stored in the data RAM section; a D/A and datareproducing section configured to convert the decoded video and audiodata into an analog video and an audio signal, respectively; and amonitor and a speaker configured to display the reproduced video dataand to reproduce the reproduced audio data, respectively; wherein theoptical head amplifier includes a variable gain amplifier, comprising: avoltage-current conversion circuit configured to output a first positivecurrent and a first negative current in proportion to a positive inputvoltage and a negative input voltage, respectively; a variable gain unitconfigured to supply four output currents obtained by processing thefirst positive current and first negative current controlled by a gainfactor A (0<A<1) controlled by a control signal, wherein the four outputcurrents include a second positive current obtained by multiplying thefirst positive current by the factor A, a third positive currentobtained by multiplying the first positive current by the factor (1-A),a second negative current obtained by multiplying the first negativecurrent by the factor A, and a third negative current obtained bymultiplying the first negative current by the factor (1-A); and avoltage output unit configured to output a positive output voltage and anegative output voltage by selectively inputting either the secondpositive current and the second negative current or the third positivecurrent and the third negative current; and a common-node feedback unitconfigured to provide a fee-back loop through the voltage output unitusing a voltage difference between the positive and negative outputvoltages and an external reference voltage.
 14. The variable gainamplifier according to claim 1, wherein the common-mode feedback unit isconfigured to ensure that the mean value of the positive output voltageand the negative output voltage is equal to the external referencevoltage Vref.